They are pretty similar! I'd say "every now and then, now and again, occasionally, once in a while, on occasion" can all be used interchangeably- to mean the occasion is rare and spread apart. "Sometimes" COULD mean the same thing but is less dramatic/ doesn't apply the events being as spread apart as the rest. For example, one could say... I usually read before bed, but sometimes I watch a show before bed, and on occasion (interchangeable with once in a while, every now and then, etc.) I just go straight to bed without doing either. Does that help? Oh and they all can be used pretty casually, but the one that would be most appropriate in formal situations too is probably "on occasion" or "occasionally." The others are more exclusively casual.

Hanji, hello! I'm K.P. and I'm glad that you are on this forum. I don't know how to write to you, so I use this question.

I understood your explanation, Zia! Why I asked this is that a Japanese English teacher wrote in his blog that you can say that "I every now and then go to the movies." instead of "I sometimes go to the movies." I had thought "every now and then" is the same "occasionally" before. I was right! Thank you, Zia!

they are all expressions that mean nearly the same thing. the only difference is that "sometimes" is used more. when you say " now and again/ every so often". it's saying, you have done it, and probably will do something again, but not soon. I hope that made sense.